Light diffusive coating and a lamp having the coating

ABSTRACT

A light diffusive coating having an excellent light diffusion effect, a method of forming the coating and a halogen lamp with the coating. The light diffusive coating includes bubbles within a continuous coating consisting of a metal oxide formed on a base. The method of forming the light diffusive coating is made by the inclusion of bubbles formed by the evaporization of a high boiling-point organic solvent within the metal oxide coating formed by the application of a combination of an organo-metallic compound with a high boiling-point organic solvent to a base surface, and the subsequent decomposition of the organo-metallic compound on baking. The halogen lamp has a porous light diffusive coating formed on the coating established on the surface of the outer vessel which allows visible light to pass through but reflects infra-red light, so that strain, caused by the difference in thermal expansion efficient of the structural material of the outer vessel and the structural material of the light diffusive layer occuring when the outer vessel is at a high temperature, is absorbed by the porous structure, and prevents peeling.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a light diffusive coating which is formed on abase surface of lamps such as halogen lamps, high pressure dischargelamp bulbs, optical filters, window glass, etc., giving a diffusivecharacteristic, and to a method of forming usch a coating, and whichmakes possible lamps which are more efficient and give uniformillumination.

2. Description of the Prior Art

In order to give uniform distribution of illumination over theilluminated surface, halogen lamps for photocopiers, for example, haveseveral filaments connected in series through conductors, and the wholeassembly is enclosed in a straight tube type vessel made of silica glassso that it lies along the axis of the tube. However, because thishalogen lamp includes a large amount of infra-red radiation in itsemissions, there is a danger that the item being photocopied will bedamaged by the heat. For this reason, the lamp surface generallyundergoes a honing treatment to provide a diffusive surface. Thismethod, however, has the disadvantage of being very laborious. Inaddition, the methods of forming a light diffusive coating by applying afine diffusive powder such as silica to the outside of the lamp byelectrostatic coating and etching may be considered. These types ofcoating are generally weak for mechanical stresses, wear easily, and insome cases the light diffusive effect falls off with a long period ofuse. Dependent on the type of fine light diffusive powder, there arecases where the coating has a poor affinity for the quartz glass, andreadily peels off.

With regard to these points, the applicants have previously developedthe technology of forming a coating on the surface of the transparentouter vessel which allows visible light to pass through, but reflectsinfra-red light, and by providing a light diffusive coating on top ofthe infra-red reflective coating, the visible light which has passedthrough the infra-red reflective coating is dispersed by the lightdiffusive coating, giving a uniform distribution of illumination on theilluminated surface. This was proposed in the Japanese PatentApplication Number Sho 58-95001/1983). However, when this technology isapplied to lamps, such as halogen lamps for example, because thetemperature of the lamp's surface is extremely high, with some lightdiffusive coatings, there is the danger that the light diffusive coatingmay peel off due to the effect of repeated switching on and off over along period. This means that it is essential to select a light diffusivecoating which is stable regardless of the rise and fall in temperatureand which is mechanically strong.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a light diffusivecoating and a method of manufacturing thereof which can be readilyapplied to large-scale production, is mechanically robust and resistantto wear, can be applied without problem to the inside or outside of thebase surface of lamps, etc., regardless of their material, and whichprovides excellent diffusion.

Another object of the invention, moreover, does not damage the effect ofApplication Number Showa 58-95001 by adding to the limits required bythe light diffusive coating proposed in the application, but aims toproduce a lamp with no danger of the diffusive coating peeling off, evenwith repeated switching on and off over a long period.

Further object of the present invention is to provide a lamp having thelight diffusive coating.

Additional objects, advantages, and features of the present inventionwill become apparent to persons skilled in the art from a study of thefollowing description and of the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a section of a halogen lamp which is one embodiment of thepresent invention;

FIG. 2 shows an enlarged schematic section of section II marked on FIG.1;

FIG. 3 shows an electron micrograph of the outer surface of the lightdiffusive coating 3 part of FIG. 2;

FIG. 4 shows a schematic section of the coating before baking in themethod of forming the light diffusive coating;

FIG. 5 shows a schematic section of a multi-layered light diffusivecoating; and

FIG. 6 is a graph showing the effectiveness of the light diffusivecoating in this embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first part of the present invention is a light diffusive coatingwhich provides light diffusion by the inclusion of bubbles within thecontinuous coating which is formed on a base.

The second part of the present invention is a method of forming thelight diffusive coating by the inclusion of bubbles formed by thevaporization of a high boiling-point organic solvent within the metaloxide coating formed by the application of a combination of anorgano-metallic compound with a high boiling-point organic solvent to abase surface, and the subsequent decomposition of the organo-metalliccompound on baking.

The third part of the present invention is a halogen lamp which has aporous light diffusive coating formed on the coating established on theouter surface of the outer vessel which allows visible light to passthrough but reflects infra-red light, so that strain, caused by thedifference in thermal expansion efficient of the structural material ofthe vessel and the structural material of the light diffusive layeroccurring when the vessel is at a high temperature, is absorbed by theporous structure, and prevents peeling.

The details of the present invention are described with reference to theembodiment in the drawings, namely, FIGS. 1 to 6. Throughout thedrawings, like reference numerals and letters are used to designate likeor equivalent elements for the sake of simplicity of explanation.

Referring now to FIG. 1, there is shown a halogen lamp for a photocopieras an example of a lamp according to the third part of the presentinvention. This halogen lamp has had a coating 2 which allows visiblelight to pass through but reflects infra-red light, formed on theoutside surface of a straight tube type vessel 1 made from atransparent, heat-resistant glass such as quartz glass. A porous lightdiffusive coating 3 has been formed on top of the reflective coating 2.Pressure sealed seal sections 4, 4 have been constructed at each end ofthe vessel 1, and molybdenum foil conductors 5, 5 have been embeddedinto each of these seal sections 4, 4. One end of each of these piecesof molybdenum foil 5, 5 is in contact with internal conductors 6, 6which are stretched inside the vessel 1. Between these internalconductors 6, 6, several filaments 7, 7 . . . are connected serially inposition along what is virtually the centre line, via non light-emittingsections formed by low resistance conductors 8, 8 . . . . The seriallyconnected filaments 7, 7 . . . and low resistance conductors 8, 8 . . .are held in position on the centre line of the vessel 1 by means ofanchors 9, 9 . . . . The other ends of the foil conductors 5, 5 areconnected to the caps 10, 10 attached to each end of the vessel 1through external conductors which are not shown in the drawing. Therequired halogen is sealed inside the vessel 1 along with an inert gassuch as argon.

Referring now to FIG. 2, which is an enlargement of section II in FIG.1, there is shown in detail the infra-red reflecting coating 2 describedabove typically has a multilayered coating, consisting of alternate highrefractive index layers 2a (left-slanting hatching) made from titaniumoxide; TiO₂ etc., and low refractive index layers 2b (right-slantinghatching), made from silica; SiO₂, etc. Due to the effect of opticalinterference, this allows visible light to pass through readily, butreflects infra-red light very well. The layers 2a, 2b are exaggerated inthe drawing; an appropriate thickness is 0.2-0.3 μm (micron meter).

Various types of the light diffusive coating 3 described above may beconsidered, but the drawings show a typical example. As is made clear byFIG. 2, and by the electron micrograph in FIG. 3, the light diffusivecoating 3 has numerous bubbles 32, 32 . . . inside the continuouscoating material 31, made from metal oxide, and numerous circular pits33, 33 . . . which are a variety of these bubbles, are formed on thesurface of the coating 31. The transmitted light is dispersed by thesebubbles 32 and pits 33, and the light diffusive layer 3 appears cloudyto the naked eye. The light diffusive layer 3 is formed with a thicknessof 3000-8000 A (angstrom), or 0.3-0.8 μm. Examples of suitable metaloxides are titanium oxide; TiO₂, silica; SiO₂, alumina; Al₂ O₃,zirconia; ZrO₂, zinc oxide; ZnO, tantalum oxide; Ta₂ O₅, tin oxide; SnO₂and indium oxide; In₂ o₃.

Even though the filaments 7, 7 of the halogen lamp are arranged withspaces between them, because their light is emitted after dispersion bythe light diffusive layer 3, there is no irregular illumination of theilluminated surface. Moreover, even though there is a large differencebetween the thermal expansion coefficients of the titanium oxide whichmakes up the light diffusive coating 3 and the reflective coating 2 andquartz glass forming the vessel 1, because the reflective coating 2 andthe light diffusive coating 3 are together extremely thin, and the upperlight diffusive coating 3 has the bubbles 32 and the pits 33 within itsmaterial 31, mechanical strain caused by the difference in thermalexpansion coefficients is alleviated, and the light diffusive coating 3does not peel off even with repeated switching on and off over a longperiod. Furthermore, because the light diffusive coating 3 is extremelythin, the level of light loss, at no more than 3-4%, is excellent. Inaddition, because the light diffusive coating 3 material is continuous,it is mechanically strong, there is no danger of wear, and there are nochanges in the light diffusive characteristics over a long period ofuse.

The manufacturing method for this halogen lamp will be outlined.Firstly, a straight tube type halogen lamp with the filaments 7, 7 . . .sealed inside is manufactured by normal methods. A titanium solution isthen prepared by dissolving an organic titanium oxide, withtetraisopropyl titanate as the main constituent, in an organic solventwith acetic ester as the main constituent. This solution has 2-10%titanium by weight, and its viscosity is adjusted to approximately 1.0cps. The sealed halogen lamp described above then has its first highrefractive index layer 2a-1, consisting of titanium oxide, formed on theouter surface of the vessel 1 by dipping it into this titanium solution,withdrawing it at a speed of approximately 30 cm/minute, drying andbaking it.

A silica solution is also prepared by dissolving an organic siliconoxide, with ethyl silicate as its main constituent, in an organicsolvent with ethyl ester as the main constituent. This solution has2-10% silicon by weight, and its viscosity is adjusted to approximately1.0 cps. The halogen lamp with the first high refractive index layer2a-1 described above then has its first low refractive index layer 2b-1,consisting of silica, formed by dipping it into this silica solution,withdrawing it at a speed of approximately 35 cm/minute, drying andbaking it. Further high refractive index layers 2a-2 . . . and lowrefractive index layers 2b-2 . . . are formed alternately in the sameway up to a total of about 10 layers, completing the construction of theinfra-red reflective coating.

5-10% of a high boiling-point organic solvent such as di-2-ethylhexylphthalate or di-octyl phthalate (abbreviated to DOP, hereinafter) isthen added to the solution of an organic titanium compound in a lowboiling-point organic solvent in the example described above, and a lowboiling point organic solvent is added to this to bring the density downto a suitable level. The halogen lamp with the infra-red coating 2described above is then dipped in this solution, withdrawn at a speed of30-50 cm/minute and dried. DOP described above is a colourless oilyliquid which boils at 231° C. and 5 mm Hg, and mutually dissolves withthe low boiling-point organic solvent to a certain extent. In the dryingprocess, only the low boiling-point organic solvent evaporates. Afterdrying, as can be seen in FIG. 4, the gel-state coating 34 is cloudedwith dispersed microscopic droplets of DOP.

The halogen lamp with the gel-state coating 34 is baked in air forapproximately 5 minutes at approximately 600 C. When this is done, theevaporation of the droplets of DOP dispersed throughout the gel-statecoating 34 and the decomposition of the organic titanium compound occurmore or less simultaneously. Because of this, the droplets of DOPevaporize in almost the same position, expand and become bubbles 32. DOPvapour in these bubbles 32 is gradually substituted by air. The dropletsof DOP near to the surface of the gel-state coating 34 burst due totheir expansion, link up with the outside air and become the pits 33.Finally a light diffusive layer 3 containing bubbles 32 and pits 33within a continuous titanium oxide coating is formed, as shown in FIG.3. As is clear from this formation process, the bubbles 32 and pits 33in the present invention are basically the same thing. In this way, inaddition to appearing to improve the formation of the organic titaniumcompound coating, DOP forms droplets within the gel-state coating 34which grow into bubbles 32. Because of this, the bubbling is effectiveeven though the gel-state coating 34 is extremely thin. Also, DOP has ahigh boiling point, close to the decomposition temperature of theorganic titanium compound, so that even when heat decomposition of theorganic titanium compound begins, sufficient vapour pressure ismaintained within the bubbles 32. This prevents the bubbles 32 frombeing crushed or the pits 33 being levelled out before the decompositionand solidification of the organic titanium compound. Accordingly, thecoating 34 hardens with the bubbles 32 and pits 33 still included and ingood shape. If too little DOP is added, the bubbles 32 and pits 33 aretoo small, and the light diffusive effect is reduced. Conversely, if toomuch DOP is added, large irregularities in the sizes of the bubbles 32and pits 33, local irregularities in diffusion, and large pits visibleto the naked eye may be formed.

If the coating 34 is made thicker by adjusting the viscosity of thecoating material, large bubbles and pits are produced, but on the otherhand, the pits 33 become shallower, and light dispersion deteriorates.In contrast to this, if the coating 34 is made thinner, then since thebubbles 32 and pits 33 remain small, it is possible to form a coatingwith several layers of this thin light diffusive coating. According toexperimentation, when a light diffusive coating 3 with a thickness of0.5-1 μm formed by several layers of thin light diffusive coating 31 wasobserved with an electron microscope, it was found to have 20,000-60,000bubbles 32 and pits 33 per 1 mm², and despite its thinness, it gaveexcellent light diffusion, and moreover there were no mechanical oroptical problems with the infra-red reflective coating 2.

The high boiling-point organic solvent is not restricted to DOP, but maybe any with similar physical properties which does not producedecomposition residues on baking.

Light diffusive coatings were made by the same method as above, byadding varying proportions of various high boiling-point organicsolvents to the organic titanium compound. The light diffusion effect ofthe coatings was investigated, and the results are shown in Table 1 innext page.

                  TABLE 1                                                         ______________________________________                                                Boiling                                                               Solvent point     5% conc.   10% conc.                                                                             20% conc.                                ______________________________________                                        DBP     339 C     Good       Good    Good                                                       diffusion  diffusion                                                                             diffusion                                DOP     231 C     Good       Good    Good                                             (5 mm Hg) diffusion  diffusion                                                                             diffusion                                DAP     I75 C     Transparent                                                                              Weak    Weak                                                                  diffusion                                                                             diffusion                                ______________________________________                                    

Here, DBP, DOP and DAP are respectively abbreviations for di-butylphthalate, di-octyl phthalate, and di-allyl phthalate.

From Table 1 it can be seen that when a light diffusive coating 3 isformed using an organic titanium compound, then 5-20% of DBP or DOPshould be added. In addition, if low light diffusion is satisfactory,then 10-20% of DAP may also be used.

Light diffusive coatings were made by the same method as above, byadding varying proportions of various high boiling-point organicsolvents to the organic silicide. The light diffusion of the coating wasinvestigated, and the results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Solvent                                                                              5% conc.     10% conc.   20% conc.                                     ______________________________________                                        DBP    Good diffusion                                                                             Good diffusion                                                                            Good diffusion                                DOP    Good diffusion                                                                             Good diffusion                                                                            Good diffusion                                DAP    Transparent  Weak diffusion                                                                            Weak diffusion                                ______________________________________                                    

From Table 2 it can be seen that when a light diffusive coating isformed using an organic silicide, then 5-20% of DBP or DOP should beadded. If formed under the same conditions then the titanium oxidediffusive coating gives better light diffusion than the silica lightdiffusive coating. This is thought to be due to the difference in theirrefractive indices.

Furthermore, the dispersive effect of the light diffusive coating can beincreased by applying several layers of a thin light diffusive coating,about 0.1 um, to the same base. In this case, the first thin lightdiffusive coating is formed on the base by the method described above,and the second and subsequent thin light diffusive coatings are formedon top. In this case, as shown in FIG. 5, the pits 33A, 33B which areproduced when the lower layers 31A, 31B are formed are covered up by theupper layers, but the bubbles 32A, 32B remain as they are. The result ofthis is as if it was a thick light diffusive coating with an extremelylarge number of bubbles, providing excellent diffusion.

When the light diffusive coating is formed in several layers, the methodused may be that of application and baking repeated for each layer, orthat of applying several layers and baking afterwards.

The light diffusion effect may be further improved by the production of0.001-10 um microcrystals by baking the continuous coating 31 in air at1,000 C. for 10 minutes after the baking and hardening process describedin the embodiment above. If this is done, the titanium layers in thepreviously formed infra-red reflective coating also crystalize, but thisis acceptable.

In addition, light diffusive coatings of different metal oxides may beformed by using organic compounds of other metals in the same way as inthe method described above. The light diffusion effect of the lightdiffusive coating 3 may also be strengthened by combination with thepre-existing technology of dispersing particles of other metal compoundswithin the light diffusive coating material 31.

Special properties may also be produced by adding metal compounds withspecial properties to these light diffusive coating materials. Forexample, by introducing powders with good thermal conductivity such asmetal powders and aluminium oxide powder, the thermal radiation isincreased. Also, if colouring powders such as cobalt blue areintroduced, a coloured light diffusive coating which colours emittedlight is produced. In these cases, the additive may or may not have alighter diffusive effect, but if such an effect is required, theparticle size of the additive should probably be 0.001 u or larger.

The operation of the halogen lamp in the embodiment shown in FIG. 1 andFIG. 2 will be explained. When electricity is passed between the caps10, 10, the filaments 7, 7 . . . emit light. Of the light emitted by thefilaments 7, 7 . . . , the infra-red light is reflected by the infra-redreflective coating 2 with most of it returning to the filaments 7, 7 . .. , heating them, and helping to improve their light-emittingefficiency. The visible light which has passed through the infra-redreflective coating 2 is dispersed by the light diffusive coating 3 andemitted as diffused light. Consequently, with this lamp, thedistribution of light on the illuminated surface is more uniform, and anoptical image of the filaments 7 does not arise, even if it is used incombination with a reflector.

Since there are many bubbles 32 and pits or variant bubbles, 33, in thelight diffusive coating 3 of this halogen lamp, if the vessel 1 of thelamp is heated to a high temperature, the strains caused by thedifference in thermal expansion coefficients of the outer tube and thetitanium oxide of the light diffusive coating are absorbed by thebubbles 32 and pits 33. Because of the light diffusive coating 3 resiststo peel off, even with repeated switching on and off over a long period.In addition, the light diffusive coating in this embodiment is extremelythin, 0.5-1 μm, which helps to reduce the strains, and is another reasonwhy peeling does not occur.

The light distribution along the axis of a photocopier lamp rated at 150V, 250 W, in the embodiment described above, before and after formationof the light diffusive coating 3 were compared. The results are shown inFIG. 6. In FIG. 6, the X axis takes the centre of the lamp as 0 andmeasures the distance along the axis in mm. The Y axis gives therelative value of illumination, taking the centre of the straight tubetype halogen lamp as 100. Curve A (solid line) shows the lightdistribution of the lamp after formation of the coating 3, and curve B(dotted line) shows the light distribution of the lamp before formationof the coating 3. From this graph it can be seen that the lightdistribution has been made uniform by formation of the coating 3.

Also, when the brightness (total luminous intensity) of the lamp beforeand after coating was compared, it fell by no more than 2-3% with theformation of the coating. This low light loss is a characteristic ofthis light diffuisive coating 3.

The presents invention can also be applied to halogen lamps, such asthose using T-shaped tube type vessels or G-shaped tube type vessels. Ina T-shaped type vessel, the filaments should be arranged almostconcentrically, and in a G-shaped type vessel they should be arrangedclose to the centre of the rounded section. In addition, this inventionmay be used to form layers of coatings which let through visible lightbut reflect infra-red light and porous light diffusive coatings in theway described above on the outer surface of both blocked and unblockedtransparent vessels. Moreover, it is possible to produce normal tubetype halogen lamps with neither infra-red reflective coating or lightdiffusive coating on the centre section.

The present invention is also applicable to halogen lamps for cars byforming layers of infra-red reflective coating and light diffusivecoating on the outer surface of a T-shaped tube type glass vessel andplacing a single filament in the centre of this vessel.

Since, even if used in combination with a reflector, no optical image ofthe filament is formed on the illuminated surface, no irregularillumination is produced by the lamp.

The present invention may also be applied to ordinary lamps. The vesselmay be either spherical or pear-shaped, and the filament should beplaced at the center of the sphere or hemisphere section. In additionthe present invention may be applied by forming layers of infra-redreflective coating and light diffusive coating on the outer surface ofan unblocked inner tube, which then has a filament placed in its center,and the assembly is then sealed in a cylindrical outer tube. In thiscase the inner tube touches the outer tube.

In the embodiment described above, a light diffusive coating 3 wasformed on the outside surface of an tube, but the invention is notlimited to this application. It may also be formed on any surface, suchas the inside surface of a halogen lamp, on the inside or outsidesurfaces of ordinary lamps and infra-red lamps, etc., using soft glasssuch as soda lime glass or hard glass such as borosilicate glass, on theinside or outside surfaces of the vessel of high pressure dischargelamps, or on plate glass such as optical filters or window glass. Here,the surfaces on which the coating may be formed are given the genericname "base".

Since the light diffusive coating in the present invention produceslight diffusion by means of bubbles included in a continuous coatingconsisting of a metal oxide formed on a base, in spite of beingextremely thin, it gives excellent light diffusion, reduces mechanicalstrain due to differences with the thermal expansion coefficients of thebase, and can withstand repeated heating and cooling shocks over a longperiod, is mechanically robust, there is no reduction in light diffusioneven over a long period of use, and light loss is extremely small.

The method of forming the light diffusive coating in the presentinvention is by applying an organo-metallic compound combined with ahigh boiling-point solvent to a base and baking so that theorgano-metallic compound decomposes to form a metallic oxide coating.Because of this, after application, the high boiling-point organicsolvent forms tiny droplets which disperse throughout the coating, andthen vaporize in the same position to form bubbles. This method allowsthe very simple formation of a light diffusive coating which isextremely thin and gives excellent light diffusion.

Furthermore, since the present invention involves the establishment of acoating which allows visible light to pass through but reflectsinfra-red light on the outside surface of the transparent vessel of ahalogen lamp, and the establishment of a porous light diffusive coatingon top of this reflective coating, there is little infra-red radiation,efficiency is high, the light distribution on the illuminated surface isuniform, and there is no danger of the appearance of an optical image ofthe filament, even if used in combination with a reflector. Moreover,there is the advantage that there is no danger of the coating peelingoff with repeated switching on and off over a long period, even thoughthe vessel may be heated at high temperatures.

What is claimed is:
 1. A light diffusive coating comprising a layer ofsolidified metal oxide containing a plurality of discontinuousair-containing void spaces formed by evaporation at approximately 600°C. of occluded solvent droplets with said layer, such that said voidspaces are sufficient in number and size to enhance light diffusion bysaid coating.
 2. A light diffusive coating according to claim 1, whereinsome of said void spaces are opened at the surface of said coating.
 3. Alamp comprising:a transparent glass vessel with an internal and anexternal surface; a light diffusive coating formed on said externalsurface of said glass vessel, wherein said light diffusive coatingcontains a plurality of discontinuous air-containing void spaces, formedby the evaporation at approximately 600° C. of occluded solvent dropletswithin said layer, such that said void spaces are sufficient in numberand size to enhance light diffusion by said coating; and a filamentsealed inside said glass vessel.
 4. A halogen lamp comprising:atransparent glass vessel with an internal and an external surface; acoating which allows visible light to pass through but reflectsinfra-red light, formed on said external surface of said glass vessel; alight diffusive coating applied onto said infra-red light reflectivecoating, wherein said light diffusive coating contains a plurality ofdiscontinuous air-containing void spaces, formed by the evaporation atapproximately 600° C. of occluded solvent droplets within said layer,such that said void spaces are sufficient in number and size to enhancelight diffusion by said coating; and a filament situated within thecentral section of said glass vessel.